Subscriber station for a bus system and method for reducing line-related emissions in a bus system

ABSTRACT

A subscriber station for a bus system and a method for reducing line-related emissions in a bus system are provided. The subscriber station includes an edge controller for symmetrizing switching edges in the bus system. The edge controller includes an element for generating a setpoint voltage characteristic on a bus in the bus system and a current mirror for transmitting the generated setpoint voltage characteristic to the bus.

FIELD

The present invention relates to a subscriber station for a bus systemand a method for reducing line-related emissions in a bus system inorder to meet the signal symmetrization requirements of the bus system.

BACKGROUND INFORMATION

The CAN bus is a differential bus system having high signalsymmetrization demands. The better the signal symmetrization, the lowerwill be the interference emission and the interferences at a subscriberstation such as a car radio, for example. Signals CAN_H and CAN_L, whichare in phase opposition, must be controlled in such a way that theirmean value deviates preferably little from mean voltage VCC5/2=2.5V.

Signal symmetrization is presently achieved by balancing the resistancesof the switches to ground GND and to potential VCC5, in such a way thatin the dominant state of the bus, in which the differential voltage ofthe signals CAN_H-CAN_L has a value of approximately 2V, the meanvoltage is 2.5V. The switch resistances are balanced out by skillfuldimensioning, for example, by adjustment or by control circuits, such asthose described in German Patent Application No. DE 10250576A1.

However, it is problematic that the symmetrization during the switchingoperation is inadequate since one switch to VCC5 (pull-up switch) andone switch to GND (pull-down switch, GND=ground) have differentcharacteristics. The total voltage at the switching point has voltagepeaks (spikes), which result in a poor emission behavior in thehigh-frequency range, a range above 1 MHz. In the related art, theseinterfering voltage peaks are suppressed by a common mode choke.However, such a common mode choke is an additional component, causingadditional costs and requiring additional space, which is usuallyavailable only to a very limited extent.

SUMMARY

An object of the present invention is to provide a subscriber stationfor a bus system and a method for solving the aforementioned problems.In particular, a subscriber station for a bus system and a method areprovided, in which a symmetrization of the switching operation, asignificant reduction in emitted interferences, an operation without acommon mode choke and symmetrization of the dominant bus state arepossible.

This object may be achieved by a subscriber station for a bus system inaccordance with the present invention. The subscriber station includesan edge controller for symmetrizing switching edges in the bus system,the edge controller including an element for generating the setpointvoltage characteristic on a bus of the bus system and a current mirrorfor transmitting the generated setpoint voltage characteristic to thebus.

The subscriber station permits better control of the currents in CAN_Hand CAN_L of the bus system statically and during the switching edgeswhile switching from the dominant state to the recessive state and viceversa.

Furthermore, the subscriber station has a high immunity to injectedinterferences, which is verifiable by a DPI test (DPI=direct powerinjection), a BCI test (BCI=bulk current injection into the cable tree).The conventional rectification effects and storage effect are things ofthe past.

Moreover, in the subscriber station, an adjustability of thesymmetrization is implementable in the final IC test (IC=integratedcircuit), which relates to the OTP programming (OTP=one-timeprogrammable (memory)). OTP programming is used for parameter andfunction adjustment of the ICs.

Another advantage of the example subscriber station is that operationwithout a common mode choke is possible.

Advantageous further embodiments of the subscriber station are describedbelow.

For example, the element for generating the setpoint voltagecharacteristic may include a Miller capacitor, which is connected to aPMOS transistor at one end and to a resistor at the other end.

It is also possible for the element for generating the setpoint voltagecharacteristic to include two power sources which are connected to aPMOS transistor.

According to one example embodiment of the present invention, the edgecontroller includes two current sources, a Miller capacitor, a PMOStransistor and a resistor. The two current sources and the Millercapacitor may be connected here to the gate of the PMOS transistor.

The current mirror may be formed using MOS low-voltage transistorsdesigned with an identical layout.

According to another option, the current mirror is connected to the busvia MOS high-voltage transistors. The MOS high-voltage transistors arecascode transistors with which an extensive isolation of the circuit ofthe edge controller from the bus is achieved.

The subscriber station may also have a reverse voltage protection diodefor protection of the circuit against a potential of the dominant levelin the bus system and a reverse voltage protection diode for protectionagainst the signal CAN_L.

The subscriber station described previously may be part of a bus system,including a bus and at least two subscriber stations, which areinterconnected via the bus in such a way that they are able tocommunicate with one another, at least one of the at least twosubscriber stations being one of the subscriber stations describedpreviously.

The object may also be achieved by a method for reducing line-relatedemissions in a bus system in accordance with the present invention. Inan example method, an edge controller generates a setpoint voltagecharacteristic for symmetrization of switching edges in the bus system,including an element for generating a setpoint voltage characteristic,and transmits this to the bus via a current mirror.

This method offers the same advantages as those mentioned above withrespect to the subscriber station.

Additional possible implementations of the present invention alsoinclude combinations, not mentioned explicitly, of features or specificembodiments described previously or hereinafter with respect to theexemplary embodiments. Those skilled in the art will also add individualaspects as improvements or additions to the respective basic form of thepresent invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is described in greater detail below withreference to the figures and on the basis of exemplary embodiments.

FIG. 1 shows a simplified block diagram of a bus system according to afirst exemplary embodiment.

FIG. 2 shows a setpoint voltage characteristic of a bus signal over timein the bus system according to the first exemplary embodiment.

FIG. 3 shows an electrical circuit diagram of a signal symmetrizationdevice of a subscriber station of the bus system according to the firstexemplary embodiment.

In the figures, the same elements or elements having the same functionare labeled with the same reference numerals unless otherwise indicated.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

FIG. 1 shows a bus system 1, which may be a CAN bus system, a CAN-FD bussystem, etc., for example. Bus system 1 may be used in a vehicle, inparticular a motor vehicle, an airplane, etc. or in a hospital, etc.

Bus system 1 in FIG. 1 has a plurality of subscriber stations 10, 20,30, each being connected to a bus 40, including a first bus core 41 anda second bus core 42. Bus cores 41, 42 may also be referred to as CAN_Hand CAN_L and are used for input of the dominant levels in thetransmission state. Messages 45, 46, 47 in the form of signals may betransmitted via bus 40 between the individual subscriber stations 10,20, 30. Subscriber stations 10, 20, 30 may be control units or displaydevices in a motor vehicle, for example.

As shown in FIG. 1, subscriber stations 10, 30 each have a communicationcontrol unit 11, a transmitting unit 12 and a receiving unit 13.However, subscriber station 20 has a communication control unit 11 and atransmit/receive unit 14. Transmitting units 12, receiving units 13 ofsubscriber stations 10, 30 and transmit/receive unit 14 of subscriberstations 20 are each connected directly to bus 40, although this is notshown in FIG. 1.

Communication control unit 11 is used to control a communication ofrespective subscriber station 10, 20, 30 via bus 40 with anothersubscriber station of subscriber stations 10, 20, 30 connected to bus40. Transmitting unit 12 is used to transmit messages 45, 47 in the formof signals and to reduce line-related emissions in bus system 1 to meetthe signal symmetrization requirements of bus system 1, as described ingreater detail below. Line-related emissions may occur on bus 40.Communication control unit 11 may be designed as a traditional CANcontroller. Receiving unit 13 may be designed as a traditional CANtransceiver with respect to its reception functionality.Transmit/receive unit 14 may be designed as a traditional CANtransceiver.

FIG. 2 shows a voltage characteristic U over time t, having switchingedges 51, 52, such as those generated by transmitting unit 12, which isshown in greater detail in FIG. 3. Switching edge 51 corresponds to atransition of the signal from dominant state 53 to recessive state 54.Switching edge 52 corresponds to a transition of the signal fromrecessive state 54 to dominant state 53. The voltage characteristicshown here has switching edges 51, 52 like a setpoint voltagecharacteristic to be generated by transmitting unit 12.

As shown in FIG. 3, transmitting unit 12 includes an edge controller 120with the aid of a simulation of Miller capacitor 121 and current sources122, an almost instantaneous current mirror 130, an output currentmirror CAN_H 140 and an output current mirror CAN_L 145.

Edge controller 120 additionally includes switching elements 123 and aPMOS transistor 124 in addition to Miller capacitor 121 and currentsources 122. Miller capacitor 121 is connected to the gate of PMOStransistor 124. Furthermore, current sources 122 are connected to thegate of PMOS transistor 124 via switching elements 123. Miller capacitor121 is connected at its other end to the drain of PMOS transistor 124. Aresistor 125 converts the voltage ramp generated at the drain of PMOStransistor 124 into a current signal for the input of a current mirror131. Resistor 124 predefines the maximum short circuit current in buscore 41 (CAN_H) and bus core 42 (CAN_L).

Current mirror 130 also includes an NMOS high-voltage cascode 132,hereinafter also referred to as an NMOS HV cascode 132, and a PMOScurrent mirror 133 for low voltage in addition to an NMOS current bank131. NMOS HV cascode 132 is connected to output current mirror 140. PMOScurrent mirror 133 is connected to output current mirror 145. Outputcurrent mirror CAN_H 140 is a PMOS current mirror for low voltage forCAN_H output current generation. Output current mirror CAN_L 145 is anNMOS current mirror for low voltage for CAN_L output current generation.

A PMOS high-voltage cascode 141, hereinafter also referred to as PMOS HVcascode 141, is connected to output current mirror CAN_H 140. PMOS HVcascode 141 is needed for an error case “short circuit of CAN_H to−27V.” In addition, a reverse voltage protection diode 142 is connectedto output current mirror CAN_H 140 for protecting the circuit againstpositive overvoltage from CAN_H. A negative potential φch_n relative tothe positive voltage supply downstream from reverse voltage protectiondiode 142 is applied to PMOS HV cascode 141.

An NMOS high-voltage cascode 146, hereinafter also referred to as NMOSHV cascode 146, is connected to output current CAN_L 145. NMOS HVcascode 146 is needed for an error case of “short circuit CAN_L to 40V.In addition, a reverse voltage protection diode 147 is connected tooutput current mirror CAN_L 145. Reverse voltage protection diode 147 isneeded in the fault case “short circuit CAN_L to −27V.” A positivepotential φch_p relative to ground is applied to NMOS HV cascode 146.

Between PMOS HV cascode 141 and reverse voltage protection diode 147,bus 40 is connected to bus cores 41, 42, which are connected to resistor143. Resistor 143 thus has the same resistance as the characteristicwave impedance of bus 40, which is why there are no reflections on bus40. Bus core 41 here stands for the transmission of signal CAN_H and buscore 42 stands for the transmission of signal CAN_L.

The circuit described above is greatly simplified with respect toresistor 143. In reality, two series-connected 60Ω resistors are presentat the end of each line of bus cores 41, 42. The respective midpoint isset at 2.5V.

In transmitting unit 12 from FIG. 3, the setpoint voltage characteristicon bus 40 is generated internally with the aid of a replica element,which includes Miller capacitor 121, current sources 122, PMOStransistor 124 and resistor 125 and is then transmitted via currentmirrors 140, 145 to bus 40. Edge controller is achieved with Millercapacitor 121, current sources 122, PMOS transistor 124 and resistor125. Current mirrors 133, 140, 145 are formed with MOS low-voltagetransistors of identical design in layout to obtain the same signaldelays and the same saturation behavior in the CAN_H and CAN_L branchesof the circuit shown in FIG. 3.

A method for reducing line-related emissions in bus system 1 is thuscarried out using edge controller 120. Edge controller 120 generates asetpoint voltage characteristic on bus 40 using an element forgenerating the setpoint voltage characteristic for symmetrization ofswitching edges in bus system 1 and transmits this voltagecharacteristic to bus 40 via current mirror 130.

The required voltage strength is achieved with the aid of cascode stepsformed from MOS high-voltage transistors, namely cascodes 132, 141, 146.

As is apparent from FIG. 3, the circuit of edge controller 120 is mostlyisolated from bus 40, which is represented by bus cores 41, 42 andresistor 143. This advantage is achieved by the cascoding transistors,namely cascodes 132, 141, 146. Therefore, injected interferences, suchas those caused by DPI, BCI, etc., are kept away from sensitive blocks,such as edge controller 120. The known rectification and storage effectsare a thing of the past.

Thus, due to edge controller 120, the same currents are present on CAN_Hand CAN_L during switching operations on bus 40, i.e., from recessive todominant or vice versa. This yields ideal or almost ideal switchingoperations having the same internal resistance on CAN_H, bus core 41 andCAN_L, bus core 42. Current sources 122, Miller capacitor 121 over PMOStransistor 124 and resistor 125 are thus matched to the switchingbehavior in combination with bus 40, so that there are only minor commonmode interferences.

According to a second exemplary embodiment, the dominant bus state issymmetrized which corresponds to dominant state 53. More specifically,the ratio of currents in the direction of output current mirror CAN_H140 and output current mirror CAN_L 145 is equalized. Thus, currentfaults in various signal paths, which could occur due to componentmismatching, are preventable. NMOS current bank 131 is advantageouslyformed adjustably. Otherwise bus system 1 is constructed as described inthe first exemplary embodiment.

All embodiments of bus system 1, subscriber stations 10, 30,transmitting unit 12 and the method described previously may be usedindividually or in all possible combinations. In particular, anarbitrary combination of features of the exemplary embodiments ispossible. In addition, the following modifications are conceivable inparticular.

Bus system 1 according to the exemplary embodiments is a CAN network ora CAN-FD network or a FlexRay network in particular.

The number and arrangement of subscriber stations 10, 20, 30 in bussystem 1 of the exemplary embodiments is arbitrary. In particular theremay also be only subscriber stations 10 or only subscriber stations 30or only subscriber stations 10, 30 in bus system 1 of the exemplaryembodiments.

Subscriber stations 10, 30 described previously and the method carriedout by them may be used to particular advantage in a modified dataprotocol, which was published on 2 May 2011 in the document “CAN withFlexible Data Rate, White Paper, version 1.0” on the websitehttp://www.semiconductors.bosch.de/ and enables, among other things, anincrease in size of the data field and a shortening of the bit lengthfor a portion of the CAN message after completed arbitration.

Subscriber stations 10, 30 represent one option for increasing thetransmission quality of CAN-FD into the range of conventional CANtransmissions by utilizing a much higher data rate for CAN-FD inparticular.

The functionality of the exemplary embodiments described previously mayalso be implemented in a transceiver, i.e., a transmit/receive unit 13,or in a communication control unit 11, etc. Additionally oralternatively, transmitting unit 12 may be integrated into existingproducts.

What is claimed is:
 1. A subscriber station for a bus system,comprising: an edge controller for symmetrization of switching edges inthe bus system, the edge controller including an element for generatinga setpoint voltage characteristic on a bus of the bus system; and acurrent mirror for transmitting the generated setpoint voltagecharacteristic to the bus, wherein: the element for generating thesetpoint voltage characteristic includes a Miller capacitor, which isconnected to a gate of a PMOS transistor at a first end and to a drainof the PMOS transistor and a resistor at a second end, the element forgenerating the setpoint voltage characteristic includes two currentsources which are connected to each other at a common node, and the twocurrent sources are connected via the common node to the gate of thePMOS transistor and to the first end of the Miller capacitor.
 2. Thesubscriber station as recited in claim 1, wherein the current mirror isformed with MOS low-voltage transistors having an identical structure inthe layout.
 3. The subscriber station as recited in claim 1, wherein thecurrent mirror is connected to the bus via MOS high-voltage transistors.4. The subscriber station as recited in claim 1, further comprising: areverse voltage protection diode for protecting against a potential of adominant level in the bus system; and a reverse voltage protection diodeagainst a signal CAN_L.
 5. A bus system, comprising: a bus; and at leasttwo subscriber stations connected to one another via the bus in such away that the subscriber stations are able to communicate with oneanother, wherein at least one of the at least two subscriber stationsincludes: i) an edge controller for symmetrization of switching edges inthe bus system, the edge controller including an element for generatinga setpoint voltage characteristic on a bus of the bus system, and ii) acurrent mirror for transmitting the generated setpoint voltagecharacteristic to the bus, wherein: the element for generating thesetpoint voltage characteristic includes a Miller capacitor, which isconnected to a gate of a PMOS transistor at a first end and to a drainof the PMOS transistor and a resistor at a second end, the element forgenerating the setpoint voltage characteristic includes two currentsources which are connected to each other at a common node, and the twocurrent sources are connected via the common node to the gate of thePMOS transistor and to the first end of the Miller capacitor.
 6. Amethod for reducing line-related emissions in a bus system, comprising:generating, by an edge controller for symmetrizing switching edges inthe bus system, a setpoint voltage characteristic on a bus of the bussystem using an element for generating the setpoint voltagecharacteristic; and transmitting the setpoint voltage characteristic tothe bus via a current mirror, wherein: the element for generating thesetpoint voltage characteristic includes a Miller capacitor, which isconnected to a gate of a PMOS transistor at a first end and to a drainof the PMOS transistor and a resistor at a second end, the element forgenerating the setpoint voltage characteristic includes two currentsources which are connected to each other at a common node, and the twocurrent sources are connected via the common node to the gate of thePMOS transistor and to the first end of the Miller capacitor.